A refrigeration system where a refrigerant is cooled by a refrigeration cycle using a compressor and an expansion turbine to cool an object, is widely known. Examples of such kind of refrigeration system include a refrigeration system having a plurality of compressors or expansion turbines arranged in series on a circulation path in which the refrigerant flows to compress or expand the refrigerant in multiple stages thereby to improve the cooling capacity, as disclosed in Patent Document 1 or Patent Document 2.